An ink-jet printing apparatus is a system which converts input image data into an output image via a liquid, i.e., ink. In this apparatus, its maintenance technique is a very important factor. Main reasons for the necessity of maintenance will be briefly explained.
(a) When input image data is printed, ink evaporates at discharge orifices which do not discharge ink among a plurality of nozzles arrayed on an ink-jet printhead. The ink viscosity in the discharge orifices increases, and no ink can be stably discharged by normal ink discharge energy, resulting in a discharge failure.
(b) During printing, ink droplets discharged from nozzles include small ink droplets (to be also referred to as a mist) in addition to main ink droplets. Small ink droplets attach around the ink discharge orifices of the ink-jet printhead, inhibiting straight ink discharge.
(c) If bubbles exist in an ink reservoir in the ink-jet printhead, gas having passed through discharge orifices and the material of the ink-jet printhead is entrapped in bubbles and grows, or bubbles expand upon temperature rise in printing. As a result, ink supply from an ink tank is inhibited, causing a printing failure.
As a maintenance technique which solves problems (a) to (c), there are known the following techniques.
(a) In accordance with the time and environment in which ink is not discharged and is left standing, a predetermined amount of ink is discharged in addition to ink discharge based on image data, and high-viscosity ink is discharged (this operation will be called preliminary discharge hereinafter).
(b) The discharge count at which ink droplets are discharged from discharge orifices is counted. When the count exceeds a predetermined value, a surface (to be referred to as a face hereinafter) of the ink-jet printhead in which discharge orifices are formed is wiped with a rubber blade or the like to remove attached ink (this operation will be called wiping hereinafter).
(c) A recovery operation is performed to suck ink from discharge orifices by using a pump and discharge ink from the discharge orifices (this operation will be called suction recovery hereinafter). In an ink-jet printing apparatus in which an ink-jet printhead and ink tank can be separated and the ink tank can be exchanged, suction recovery is executed even after the ink tank is exchanged.
The wiping operation and suction recovery operation will be briefly explained with reference to the accompanying drawings.
FIG. 1 is a view for explaining the wiping operation. Reference numeral 1101 denotes a rubber blade which wipes; 1102, a face to be wiped; 1103, an ink discharge orifice (ink discharge nozzle); 1104, attached ink which inhibits discharge; and 1105, a wiring direction. Wiping is to, while pressing the rubber blade 1101 against the ink-jet printhead, move the rubber blade 1101 in the direction 1105, bring the blade into contact with the attached ink 1104, and wipe the attached ink 1104 from the face, as shown in FIG. 1.
FIG. 2 is a view for explaining the suction recovery operation. Reference numeral 1201 denotes an ink-jet printhead; 1202, an ink discharge nozzle; 1203, a face; 1204, a suction cap; 1205, an ink discharge tube; and 1206, a suction pump which generates a negative pressure for sucking ink. In suction recovery, the suction cap 1204 generally made of rubber is abutted or pressed against the face 1203 and tightly contacts with it. The suction pump 1206 is pivoted in a direction indicated by an arrow 1207 to generate a negative pressure. Ink in the ink-jet printhead 1201 is sucked from the ink discharge nozzle (ink discharge orifice) 1202 into the suction cap 1204, and discharged from the ink discharge tube 1205.
In recent ink-jet printing apparatuses to which higher image qualities and higher speeds are required, the number of types of inks used and the number of discharge orifices for discharging ink abruptly increase from those several years ago. In this situation, the maintenance technique becomes more important.
An increase in image quality of recent ink-jet printing apparatuses will be explained in short.
The ink-jet printing apparatus is originally configured to form an image by superposing images of three primary colors by so-called subtractive color mixture of cyan ink, magenta ink, and yellow ink.
In addition to these three color inks, black ink capable of expressing a high contrast, and light inks (light cyan ink and light magenta ink) prepared by decreasing the content of a coloring material in order to improve tone reproduction are used. Also, a technique of minimizing discharge ink droplets in order to reduce graininess of an output image is introduced. These measures make it possible to form a high-quality image.
In order to further increase the image quality, a special ink (color other than cyan, magenta, and yellow) for expressing a color gamut which cannot be reproduced by the above-mentioned six color inks is used. A color pigment ink which improves conservation of an output image is used. There is also known a liquid which improves glossiness by applying the liquid before or after discharging ink to a printing medium.
An example of increasing the image quality, there is known an ink-jet printing apparatus in which inks of orange and green for widening the reproducible color gamut are mounted in addition to inks of black, cyan, magenta, yellow, light cyan, and light magenta (see Japanese Patent Application Laid-Open No. 2001-138552).
As described above, only one suction cap 1204 is used as shown in FIG. 2 in maintenance technique (c) when various types of inks are employed to increase the image quality. If the number of ink types is, e.g., eight, suction recovery is executed for all ink tanks of the eight colors every time an ink tank of one color is exchanged, excessively consuming ink.
As a method of solving this problem, the ink discharge nozzle building portion in one ink-jet head 2001 is divided into a plurality of nozzle portions, e.g., a first nozzle portion 2003, second nozzle portion 2005, . . . , as shown in FIG. 3. The respective nozzle portions are independently equipped with suction caps 2007, 2009, . . . . The count and timing at which the suction recovery operation is performed can be changed for the respective nozzle portions 2003, 2005, . . . .
This arrangement can minimize a redundant ink amount which is consumed in exchanging an ink tank, suction recovery at an early timing, or the like. The total consumption of ink in the whole apparatus including the consumption of ink in suction recovery can be reduced.
However, when the number of ink tanks which store ink to be supplied to divided discharge nozzle portions, the ink supply channel (pipe structure for supplying ink) extending from the ink tank to the ink discharge portion, and the like change between divided discharge nozzle portions, an optimal negative suction pressure and ink flow rate necessary for suction recovery at each discharge nozzle portion may change.
Even if a suction cap is prepared for each nozzle portion, only one suction pump is generally arranged to avoid increases in size and cost of the apparatus. In this case, ink discharge tubes connected to respective suction caps are connected to the same suction pump. The ink discharge tubes which connect the corresponding suction caps to the suction pump are identical (same diameter, same material, and the like), so negative pressures and ink flow rates which are generated upon driving the suction pump once are equal to each other. To perform the recovery operation at the respective discharge nozzle portions in this arrangement, an air communication valve between the suction pump and each cap is opened/closed to switch the ink discharge tube connected to the suction pump and perform suction recovery sequentially for the respective discharge nozzle portions.
For this reason, when the optimal negative suction pressure and ink flow rate that are necessary for suction recovery change between the discharge nozzle portions, suction recovery must be sequentially done under pump driving conditions optimal for each discharge nozzle portion in order to generate an optimal negative suction pressure and ink flow rate at each discharge nozzle portion. The time taken for suction recovery becomes long in accordance with the number of discharge nozzle portions, and the user suffers extra stress.